68 
THE STEM-ANATOMY OF 
I will now proceed to describe some points of the structure of the 
Euphorbia and Aloe from Namaqualand above-mentioned. 
Euphorbia virosa Willd. 
The numerous elongated branches of this candelabra-like type spring from 
near the base of the much thicker main stem which in a young state is 
lateral 1 . 
In a section, either transverse or radial, through the stem it can be seen 
that the pith is traversed by very large air-cavities which are really con- 
tinuous throughout the entire tissue. The size, shape, and distribution of 
these cavities, as seen in radial longitudinal and transverse sections of the pith, 
can be at once ascertained by reference to Photograph 2 2 and Figs. 2, 4, 5. 
Such a medullary structure is surely unheard of in any Euphorbia-stem 
hitherto investigated 3 ! 
I may now refer to the mode of origin of these remarkable air-spaces in 
the species under discussion. 
In the apical region of the pith there is no sign of them whatsoever. A 
little lower down they begin to arise in the form of larger and smaller, lighter 
or more transparent areas, as seen in transverse section (Fig. 1, as). 
The ordinary intercellular air-spaces in the pith are exceedingly small. I 
could discover absolutely no indication that the big air-cavities arose schizo- 
genously in the first instance by the enlargement of the intercellular spaces. 
On the other hand, it is clear that we must attribute to these air-cavities a 
lysigenous origin, for in no other way can the appearance of the transparent 
areas in the midst of the dense medullary tissue be explained. Moreover, I 
have ascertained by direct observation that the cavities arise by the breaking- 
down of groups of cells at various points throughout the pith. The walls of 
the cells are dissolved probably by fermentative action, for there is no evidence 
of their mucilaginous degeneration. Fig. 3 shews the formation of one of the 
small cavities such as are seen in Fig. 1 ; its lysigenous mode of origin is 
fairly apparent. The larger cavities arise by the progressive destruction of 
the cells at the periphery. All the cells have protoplasmic contents; their 
walls are not perforated by the large circular pits which are so characteristic 
of the cells of some types of water-storing tissue. 
The mode of origin of these air-cavities is thus explained. It is, however, 
quite another matter to determine the function which they subserve, and this 
feat is quite beyond my powers. 
1 Pearson, Ann. Bol. Herb, i (1914), Photograph 1. 
2 Pearson, l.c. 
3 Cf. Warming: The (Ecology of Plants : — “ True water-tissue is thin-walled, contains water 
but no chlorophyll, is devoid of intercellular spaces (as no gaseous exchange occurs in it) and its 
cells are usually very large.” 
